The statements in this section merely provide background information related to the present disclosure and do not necessarily constitute prior art.
As higher-speed quantum key distribution systems (QKD systems) are introduced, the speed of generating and detecting a quantum signal is increasing. The inventor(s) noted that to increase the final key generation speed, it is necessary to have not only a fast signal processing circuit but also an efficient signal processing scheme. The inventor(s) also noted that there are factors irrespective of the performance of the signal processing circuit, such as an idle time for waiting until information is completely transmitted and received through a public channel and the order of pre-processing and post-processing using the shared information, but these factors makes a total quantum information processing time longer than a quantum information transmission and detection time.
The inventor(s) experienced that when quantum information is transmitted, since the number of times that a receiver detects a photon signal is very small compared to a system clock, received information subject to signal processing is not easily arranged in correct order. Furthermore, the inventor(s) experienced that considering that, for basis comparison and decoy signal processing, information generated by both a transmitter and a receiver should be stored until signal processing is completed, transmitting and performing signal processing on quantum information in frame units is advantageous. A frame refers to a lump of information considered as a unit when the phase or polarization of bit/basis information is modulated, the amplitude of decoy information is modulated, and modulation information is stored for subsequent signal processing.
The inventor(s) noted that when frame-unit quantum information is transmitted, signal processing should be completed within a frame cycle. However, The inventor(s) experienced that processing within a cycle may not actually be achieved in consideration of a time taken for communication and signal processing. The inventor(s) experienced that consequently, key generation speed of the system is reduced and bandwidths of a quantum channel and a public channel are not efficiently utilized.
For example, U.S. Pat. No. 7,539,314 discloses a QKD system in which a transmitter generates and transmits a frame signal, a receiver completely processes the frame signal and then transmits a ready signal to the transmitter, and the transmitter checks the ready signal and then transmits a subsequent frame.
The inventor(s) has noted that according to such QKD system, since the transmitter does not transmit a frame-unit quantum signal before a ready signal is received from the receiver, overflow in signal processing of the receiver is prevented. However, the inventor(s) experienced that in terms of operation of the whole system including the transmitter and the receiver, a ratio of actual information exchange compared to operation speed is low and thus key generation speed is reduced.